Mary, Queen of Scots, her cipher and the Babington Plot

On 8 February 1587, at Fotheringhay Castle, Mary Stuart, former Queen of Scots, was beheaded after almost twenty years of captivity in England. The verdict that condemned her rested largely on ciphered letters, intercepted and read by the men of Francis Walsingham, Elizabeth I's Secretary of State. It is one of the most famous cases in which the decipherment of a message decided a person's fate.

Letters in beer barrels

In 1586 Mary was held at Chartley, in Staffordshire, and all correspondence was forbidden to her. A young Catholic, Gilbert Gifford, offered her a secret channel: letters would enter and leave the house hidden in a watertight container inside the beer barrels supplied by a local brewer. Mary did not know that Gifford was working for Walsingham, and that every letter was copied before reaching its destination.

The nomenclator and Thomas Phelippes

Mary's correspondence used a nomenclator, the most common system in the chanceries of the time: an alphabet of symbols for individual letters, some meaningless nulls to confuse, and special symbols for frequent words and important names. It was an enriched monoalphabetic substitution, secure only as long as no one had much material to compare.

Walsingham entrusted the letters to Thomas Phelippes, an expert in languages and ciphers. Using frequency analysis and the help of probable words, Phelippes reconstructed the nomenclator. When Anthony Babington, a young Catholic gentleman, wrote to Mary with a plan for her release and the killing of Elizabeth, the captive queen's reply, of July 1586, was read almost in real time.

The forged postscript

Mary's letter approved the plan, but Walsingham also wanted the names of the conspirators. Phelippes therefore added to the letter, using the same cipher, a forged postscript asking Babington to give the names of the six gentlemen charged with the assassination. The draft of this postscript survives. Babington and his companions were arrested and executed in September 1586.

At her trial Mary denied having approved the assassination, arguing that her secretaries might have altered the texts. But the deciphered documents, together with the testimony of the secretaries themselves, were enough for the commission. The lesson was bitter: a weak cipher is more dangerous than no cipher at all, because it leads the writer to speak with a frankness he would never use in plain text.

The letters found in 2023

The story of Mary Stuart's ciphers has had a recent sequel. In 2023 a team of three researchers announced that they had identified and deciphered, in the collections of the Bibliothèque nationale de France, dozens of ciphered letters written by Mary between 1578 and 1584 to the French ambassador Michel de Castelnau. The texts, wrongly catalogued for centuries, offer new information about the years of her captivity.

The case of Mary Stuart is often cited as an example of the fragility of nomenclators. But it is also a story of people and betrayed trust: the brewer, the double agent, the decipherer. Cryptography here is not a game of letters but a decisive part of the politics of a kingdom and of the life of a queen.